During the American Revolutionary War he was one of the founders of and served with the First Troop Philadelphia City Cavalry.

When the Continental Congress sought to raise money for the army in 1780, McClenachan and Robert Morris were the two highest subscribers to the effort.

In 1796, he defeated Robert Waln to win election to the 5th United States Congress from Pennsylvania's 2nd congressional district.

After his service in Congress, his businesses failed and a fraudulent transfer of his assets to his children resulted in McClenachan losing much of his fortune and led to his imprisonment for debt.

In 1802, he wrote to President Thomas Jefferson, seeking the position of Purveyor of Stores.